1. Field of the Invention
The present invention relates to a technique for vapor growth of a boron-phosphide-based semiconductor layer which exhibits both of excellent surface flatness without microcracks and of excellent continuity on the surface of an underlying layer such as a single-crystal silicon substrate.
2. Description of the Related Art
Boron phosphide (BP) has been traditionally known as a Group III-V compound semiconductor (see “An Introduction to Semiconductor Device” by Iwao Teramoto, first edition, published by Baifukan Co., Ltd. on Mar. 30, 1995, p. 28). A crystal layer formed of a boron-phosphide-based semiconductor containing boron (B) and phosphorus (P), such as boron phosphide, has been employed as a buffer layer constituting a light-emitting device (see U.S. Pat. No. 6,069,021). Such a crystal layer has also been employed as a contact layer for forming an ohmic electrode (see Japanese Patent Application Laid-Open (kokai) No. 2-288388). Such a crystal layer has also been employed for forming a boron phosphide (BP)/gallium aluminum nitride (GaxAl1−xN: 0≦X≦1) superlattice layer serving as an active layer (light-emitting layer) of a laser diode (LD) (see Japanese Patent Application Laid-Open (kokai) No. 2-288388). The aforementioned superlattice layer formed of boron phosphide and a Group III nitride semiconductor layer containing nitrogen (N) has also been employed as a cladding layer for a light-emitting layer (see Japanese Patent Application Laid-Open (kokai) No. 2-288388).
A boron-phosphide-based semiconductor layer is formed on, for example, a single-crystal silicon substrate by a vapor-phase growth technique such as metal-organic chemical vapor deposition (MOCVD) (see U.S. Pat. No. 6,069,021). When boron phosphide is formed through MOCVD, triethylboran ((C2H5)3B) and phosphine (PH3), for example, are employed as raw materials (see U.S. Pat. No. 6,069,021). Boron phosphide has been known to be formed through halide vapor phase epitaxy employing halogenated compounds such as phosphorus trichloride (PCl3) and boron trichloride (BCl3) (see (1) J. Crystal Growth, 13/14 (1972), pp. 346-349 and (2) J. of Jpn Association of Crystal Growth, Vol. 25, No. 3 (1998), A, p. 28). Also, boron phosphide has been formed through hydride vapor phase epitaxy employing diborane (B2H6) and phosphine (see (1) J. Appl. Phys., 42 (1) (1971), pp. 420-424 and (2) J. Crystal Growth, 70 (1984), pp. 507-514).
Conventionally, a vapor-phase growth reactor formed of, for example, quartz has been employed for the formation of a boron-phosphide-based semiconductor layer on a single-crystal silicon substrate (see (1) “Handotai Gijutsu (Jo)” authored by Katsufusa Shohno, ninth printing, published by University of Tokyo Press on Jun. 25, 1992, pp. 74-76). A single-crystal silicon substrate is firstly placed on a susceptor provided in a vapor-phase growth reactor, and subsequently the susceptor or the silicon substrate is heated to a temperature suitable for formation of a boron-phosphide-based semiconductor layer. The temperature suitable for formation of a boron-phosphide-based semiconductor layer is known to be, for example, 900° C. to 1,250° C. (see the aforementioned “Handotai Gijutsu (Jo),” p. 76). Thereafter, through a conventional vapor-phase growth method for a boron-phosphide-based semiconductor layer, a gas source of a Group III element such as boron and a gas source of a Group V element such as phosphorus are fed into the vapor-phase growth reactor, to thereby initiate formation of a boron-phosphide-based semiconductor layer (see Inst. Phys, Conf. Ser., No. 129 (IPO Pub. Ltd., 1993, UK), pp. 157-162). Hydrogen gas (H2) is employed for carrying the above gas sources into the vapor-phase growth reactor (see the aforementioned Inst. Phys, Conf. Ser., No. 129).
Meanwhile, a film of silicon nitride (Si3N4) or silicon dioxide (SiO2) present on the surface of a single-crystal silicon substrate is known as a masking material to prevent growth of, for example, gallium nitride (GaN) thereon (see (1) J. Crystal Growth, 230 (2001), pp. 341-345 and (2) J. Crystal Growth, 230 (2001), pp. 346-350). By virtue of the aforementioned effect, a silicon nitride film or silicon oxide film is employed for masking the substrate so as to form a nitrogen-containing Group III nitride semiconductor layer on a selected region of the surface of the substrate (see “Group III Nitride Semiconductor,” first edition, published by Baifukan Co., Ltd. on Dec. 8, 1999, pp. 122-124).
When the aforementioned BP/GaxAl1−xN (0≦X≦1) superlattice layer is formed through a conventional technique for formation of a boron-phosphide-based semiconductor layer, BP crystal layers and GaxAl1−xN (0≦X≦1) crystal layers are stacked alternately in the same vapor-phase growth reactor. When such a superlattice layer containing a nitrogen-containing Group III nitride semiconductor layer is formed in the same vapor-phase growth reactor, decomposition products containing Group III nitride semiconductor crystals are deposited onto the inner wall of the reactor, or a susceptor provided in the reactor. Nitrogen constituting the Group III nitride semiconductor layer readily vaporizes at a temperature of about 1,000° C. or above at which a boron-phosphide-based semiconductor layer has been conventionally formed (see J. Phys. Chem., 69 (10) (1965), pp. 3455-3460). Therefore, when the temperature of the vapor-phase growth reactor is raised so as to form a boron-phosphide-based semiconductor layer, nitrogen is released from the decomposition products (deposits) containing the Group III nitride semiconductor crystals in the vapor-phase growth reactor. Particularly, the sublimation temperature of indium nitride (InN) is as low as about 620° C. under vacuum (see “Compound Semiconductor Device” edited by Japan Industrial Technology Association, New Material Technology Committee, published by Kogyo Chosakai Publishing Co., Ltd. on Sep. 15, 1973, p. 397). Therefore, when the decomposition products contain indium nitride, considerable amounts of nitrogen atoms are released from the deposits in the vapor-phase growth reactor.
A portion of the nitrogen atoms released in the vapor-phase growth reactor at high temperature reacts with silicon at the surface of a single-crystal silicon substrate, to thereby form a silicon nitride film on the substrate. The thus-formed silicon nitride film impedes proper formation of the aforementioned Group III nitride semiconductor layer, as well as proper formation of a boron-phosphide-based semiconductor layer. Consequently, the resultant boron-phosphide-based semiconductor layer has a rough surface and exhibits discontinuities. When the boron-phosphide-based semiconductor layer formed on the surface of the single-crystal silicon substrate does not have the requisite surface flatness or continuity, a crystal layer exhibiting continuity and excellent surface flatness cannot be formed on the boron-phosphide-based semiconductor layer. When a light-emitting device, such as a light-emitting diode (LED), is produced from a stacked layer structure including a discontinuous crystal layer, the resultant LED exhibits a low forward voltage (i.e., Vf) and poor rectification characteristics, because of discontinuity of the crystal layer or non-flatness of a pn-junction interface.